Electrical stimulation of the body for medical purposes is well known in the prior art. An example of a device for this purpose is the well-known cardiac pacemaker. In the pacemaker context, as well as other body stimulation contexts, the stimulation is delivered to the desired body site by an electrode carrying lead.
Interactions between the lead and body can vitiate the desired effects of the stimulation. For example, material reactions and healing may encourage fibrosis. In the pacemaking context, fibrosis is believed to be a major factor in the increase in chronic stimulation threshold that is usually experienced. Also, mechanical trauma may result in inflammation of the tissue to be stimulated. Such inflammation may alter the response of the tissue to the stimulation energy, both acutely and chronically.
Other interactions between the lead and body, while not directly affecting the response of the tissue to the stimulation energy, can result in the occurrence of undesirable events. For example, the placement of a pacemaking lead may induce a cardiac arrhythmia. Furthermore, the presence of the lead may also promote thrombus formation.
The interactions noted above have long been recognized and efforts made to ameliorate their consequences. For example, the lead may be configured to reduce mechanical trauma and the response of irritable tissue during lead placement. Materials may be selected for the lead body and electrodes to minimize fibrosis. However, lead configuration must take into account, other factors such as the efficiency of the delivery of the stimulation energy, the ease of lead placement, maintenance of the desired electrode position and reliability of the lead over extended period, of time. An accommodation of these interests has resulted in leads whose configuration necessarily results in undesirable interactions between the lead and body.
The use of therapeutic drugs released in vivo to counter trauma caused by an implanted device such as a cardiac pacemaker lead is well known. Such trauma typically occurs in the region of in which the distal end of the pacing lead contacts the cardiac tissue.
Tined pacing leads which have a cavity or collar at the distal end of the lead containing a drug to counter undesirable interactions between the lead and tissue are disclosed in U.S. Pat. Nos. 4,711,251 and 4,506,680 to Stokes; U.S. Pat. No. 4,844,099 to Skalsky et al.; and U.S. Pat. No. 4,972,848 to Di Domenico et al. Di Domenico et al. Discloses a drug compounded within a polymer matrix which is specifically chosen to be dimensionally stable, not expanding when hydrated. They suggest that the degree of crosslinking of the polymer may be useful in varying the elution rate. The Stokes '680 patent teaches a porous electrode design that houses a plug of swellable material containing a drug.
Steroid-eluting leads having a tip electrode housing a variety of matrix materials with a drug being stored in, and dispensed from, the tip electrode, are disclosed in U.S. Pat. No. 4,819,662 to Heil, Jr. et al.; in U.S. Pat. No. 4,606,118 to Cannon et al.; and in U.S. Pat. No. 4,577,642 to Stokes.
A screw-in pacemaker lead is disclosed in U.S. Pat. No. 4,819,661 to Heil, Jr. et al., which has a chamber open to the distal end of the lead. A matrix impregnated with a therapeutic drug is retained in the chamber. A screw-in lead with a drug impregnated matrix is disclosed in U.S. Pat. No. 4,953,564 to Berthelson in which the drug elutes out by means of a porous sintered elution path. Because the matrix swells in use, an expansion space is provided.
An implantable, porous stimulating electrode with a thin coating of hydrophilic polymer in which is embedded an anti-inflammatory steroid is disclosed in U.S. Pat. No. 5,103,837 to Weidlich. In this system, the steroid simply diffuses from the polymeric layer into the adjoining tissue to reduce growth of connective tissue.
U.S. Pat. No. 4,711,251 to Stokes, mentioned above, includes an embodiment that uses an osmotic pump to control dispensing of the drug. It has two chambers separated from each other by an impermeable membrane. The inner chamber contains the drug and is adjacent the electrode; the outer chamber is separated from body fluids by a semi-permeable membrane. According to the specification, body fluids will enter the outer chamber through the semi-permeable membrane to impart a pressure on the inner chamber via the impermeable membrane, resulting in dispensing of the drug stored within the inner chamber through the electrode. However, it is unclear why body fluids would enter the outer chamber to the extent that pressure would be imparted to the inner chamber. Stokes does not describe anything within the outer chamber that would draw in fluid. A figure shows a fluid in the outer chamber; presumably, this is body fluid which has entered the outer chamber after implantation, since no other outer chamber fluid is mentioned.
U.S. Pat. No. 5,496,360 to Hoffmann et al. describes an electrode that contains at least two matrix devices. One has a principal function of drawing in body fluid, the other has the function of releasing a drug like a steroid, for example, dexamethasone sodium phosphate. This patent presents a concept that may not work. The size of the bore and the electrode porosity are not defined and, therefore, more likely than not, the surface tension of body fluid/water across the opening/open bore, as illustrated will restrict access as a bubble forms. This typically occurs with solid electrodes containing a single opening. Further, the Hoffmann et al. electrode would be very expensive to fabricate. In contrast, the electrode of the present invention is extremely easy to machine and assemble and very cost effective by comparison.
It was with knowledge of the foregoing state of the technology that the present invention has been conceived and is now reduced to practice.